You appear to be knowledgable in this field. I am a motorcycle rider and for 44yrs I have strived to find waterproofs. I have tried all types of motorcycle gear and outdoor waterproofs. Also ski gear and as yet have found nothing to keep me dry in heavy rain ???. HELP if possible
If membraned shells arn’t working for you, have you looked at Paramo kit. Technically it’s not meant to be waterproof, but people find it more waterproof and breathable than any thing else (outside of a laboratory). For really terrible weather, if I wanted to guarantee to stay warm and comfortable, I’d probably wear a Buffalo S6 mountain shirt and pants (designed for the SBS) next to my skin, with a standard good quality biking layer over the top. Montane also do Pertex and Pile kit, but it’s sized in a different way, and so isn’t so good as a next to skin system worn under something else. Buffalo would be great in bad weather, but not in hot weather (but then you won’t need it). Staying dry in heavy weather AND going fast on a bike is impossible unless your wear a drysuit with latex cuffs and neck (not that comfy!), but staying comfortable is easy - just us Buffalo kit next to your skin (have a read of this article to see why soft shell works. An alternative would be a one piece fibre pile suit (a Wooly bear suit) worn next to your skin, and you can get one from a company like Brenig for about £40, or a little more if you want it custom made.
Hope that helps.
Note: Image above from Top Gear article on motorbike waterproofs.